A magnetic recording medium having insufficient lubricating properties generally causes an increased coefficient of friction in its magnetic layer because the surface of the magnetic layer is made smooth by contacting with tape transport systems of magnetic recording devices. The magnetic recording medium having an increased coefficient of friction of magnetic layer adheres to the tape transport systems of the magnetic recording devices through small water drops, particularly at high humidity (i.e., more than about 60% of relative humidity). In this case, if the running tension is more than the adhering strength, the medium runs under the condition of stick slip and makes a running noise which is audible. A recording signal of a magnetic recording medium which runs under the stick slip condition causes a frequency modulation in the running direction of the medium, and, therefore, normal reproduction of the recorded signal is difficult. Where the recorded medium runs under the stick slip condition in the reproduction operation, normal reproduction is difficult because wow flutter and/or jitter occurs. On the other hand, if the adhering strength is more than the running tension, recording and reproduction operations are impossible because the medium does not run normally and causes so-called "tape squeal". Magnetic recording media and magnetic recording devices which cause such tape squeal are of no commercial value.
The tape squeal depends on materials, running tensions or running speed of tape transport systems in magnetic recording devices. It also depends on the increase of surface smoothness or surface abrasion coefficient of the magnetic recording medium.
Various solid and liquid lubricants have been employed to improve the surface lubricating properties of magnetic recording medium. Examples of such lubricants include inorganic or organic fine particles (.alpha.-Al.sub.2 O.sub.3, graphite, silica, Cr.sub.2 O.sub.3, ZnO, carbon black, etc.), and organic surface active agents (higher hydrocarbon compounds, aliphatic alcohols, fatty acids, fatty acid esters, fatty acid amides, fatty acid salts, quaternary salts of fatty acids, etc., which may contain any number of carbon atoms and preferably have a boiling point of more than about 100.degree. C. and a melting point of less than about 150.degree. C.). These lubricants are generally used in an amount of 0.1 to 20 parts by weight per 100 parts by weight of binders.
However, even if these lubricants are used, a magnetic recording layer having satisfactory properties is not necessarily obtained. For example, if a large amount of the lubricants is used, the mechanical strength of the resulting magnetic layer is often lowered or, in some cases, they gradually come out on the surface of the magnetic layer. Further, the ferromagnetic particles are not sufficiently dispersed in the presence of a large amount of these additives. For example, when glycerol tristearate as disclosed in German OLS No. 3,210,417 and Japanese Patent Application (OPI) No. 119933/81 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") is added as a lubricant to a magnetic layer in an amount sufficient to prevent the tape squeal, such amount of glycerol tristearate adversely affects electromagnetic properties of the resulting magnetic layer and reduces the S/N ratio. Also, butyl palmitate which is also a typical example of lubricants as disclosed in Japanese Patent Application (OPI) No. 53402/74 is added to a magnetic layer in an amount sufficient to provide satisfactory tape running properties against stainless steel guide poles of magnetic recording devices, such amount of butyl palmitate often causes tape squeal at a magnetic head of the devices. Further, an anhydrosorbitol tri-fatty acid ester and an anhydrosorbitol mono-fatty acid ester have been known as lubricants as disclosed in Japanese Patent Publication No. 31442/72, but these esters lack adaptability to the manufacturing processes (e.g., high speed coating) of the magnetic recording medium because they react with isocyanates causing an increase in the viscosity of magnetic coating compositions with the passage of time.